1. Field of the Invention
The present invention relates to a device for controlling a flow rate of working oil supplied from a hydraulic pump mounted in a construction machine such as a hydraulic excavator to a hydraulic actuator.
2. Description of the Related Art
As a device for controlling a flow rate of working oil supplied from a hydraulic pump mounted in a construction machine such as a hydraulic excavator to a hydraulic actuator, a technique described in Japanese Patent Laid-Open No. Hei8-93705 is known for example. A flow rate control valve for transferring a part of discharge oil of the hydraulic pump to a tank controls the flow rate of the working oil in a flow path so as to maintain a constant differential pressure between a discharge pressure thereof and a load pressure of the hydraulic actuator. In the above flow rate control, irrespective of a change in a load of the hydraulic actuator, it is possible to supply the working oil at a flow rate required for the actuator with avoiding waste.
FIG. 3 shows a conventional example of the device for performing such flow rate control.
Here, a control valve 3 is a pilot switching valve lying between a variable capacity type hydraulic pump 1 and a hydraulic actuator (a hydraulic cylinder in the figure) 2. The control valve 3 is activated for opening and closing so that areas of an opening 3a for meter-in flow rate control and an opening 3b for meter-out flow rate control are changed, and the action thereof follows an operation of a remote control valve 4.
A flow rate control valve 6 is lying between a pump discharge line 5 connecting the hydraulic pump 1 and the control valve 3 and a tank T, and activated for opening and closing so as to change a flow rate of working oil returned from the line 5 to the tank T. In the flow rate control valve 6 serving as a pilot operation type flow rate control valve provided with two pilot ports, to one of the pilot ports is inputted a discharge pressure of the hydraulic pump 1 as a pilot pressure, and to the other pilot port is inputted a pressure on the downstream side of the opening 3a of the control valve 3 (a load sensing pressure) as the pilot pressure through a load sensing pipe 11. The flow rate control valve 6 is opened in accordance with a differential pressure between both the pilot pressures. At a flow rate corresponding to the differential pressure, discharge oil of the hydraulic pump 1 is returned to the tank T.
An operation pressure corresponding to an operation amount of an operation lever outputted from the remote control valve 4 serves as the pilot pressure of the control valve 3 and activates the control valve 3. Meanwhile, a pressure value thereof is detected by a pressure sensor 7 and inputted to a controller 8. By inputting a control signal to a regulator 9 installed with the hydraulic pump 1, the controller 8 controls a discharge flow rate of the hydraulic pump 1.
In the device of FIG. 3, generation of shock at the time of starting, that is, at the time of opening the control valve 3 is a problem.
Specifically, when the remote control valve 4 is not operated and the control valve 3 is located at a neutral position, the control valve 3 interrupts between the hydraulic pump 1 and the hydraulic actuator 2. Meanwhile, the controller 8 performs control for suppressing the discharge flow rate of the hydraulic pump 1 to a minimum flow rate. At the time, since the pressure on the downstream side of the opening 3a for meter-in flow rate control (the load sensing pressure) is lower than a pump pressure, the flow rate control valve 6 is fully opened. Mainly through the fully opened flow rate control valve 6, the discharge oil of the hydraulic pump 1 is transferred to the tank T.
In such a state, when the lever of the remote control valve 4 is operated and the control valve 3 is opened, the pressure on the downstream side of the opening 3a (the load sensing pressure) is instantaneously raised up to a pressure corresponding to a load pressure of the hydraulic actuator 2. The above pressure radically activates the flow rate control valve 6 in the closing direction. Since the radical closing action of the flow rate control valve 6 radically raises the pump pressure, a sense of the shock is easily given to an operator or the like. Particularly, in the case where control for suppressing the discharge flow rate of the hydraulic pump 1 to the minimum flow rate (a standby flow rate) is performed when the control valve 3 is located at the neutral position as mentioned above, the discharge pressure of the hydraulic pump 1, that is, the pump pressure is low. In such a state, when the lever of the remote control valve 4 is operated, the load sensing pressure is rapidly raised and quickly exceeds the pump pressure. Therefore, a differential pressure thereof suddenly activates the flow rate control valve 6 in the closing direction. Moreover, in the above closing action, overshoot is easily caused due to delay of sensing a rise in the load pressure of the actuator by the flow rate control valve, and an inertial force of a valve body itself of the flow rate control valve 6. The above overshoot in the closing action causes a significant change of the pump pressure. Therefore, although the lever of the remote control valve 4 is slowly operated, there is a fear that a significant sense of the shock is given to the operator or the like.
It should be noted that the radical change of the pump pressure can be eased to some extent by designing so as to slow down a response characteristic of the flow rate control valve 6 for example. However, at the time of operation where a radical increase of the actuator flow rate is demanded, such a setting of the response characteristic prevents satisfying the demand quickly.